58 



We have seen that vou Bugh refers these beds to eontraction of the substance of the granite 

 at a time when ithad a degree of consistance » which in most cases was far removed from the 

 conditioa of fluidity." But may not the internal structure of granite and other hypogene rocks 

 be primarily due, not to the merely mechanical effect of eontraction on cooling, but to the original 

 conditions of crystallization ? Reversing the Neptunian theory of the deposition of the primary 

 rocks from a state of chemical solution in a hot fluid upon the upper surface of the earth's 

 crust, may or rather must we not conceive them to have been gradually deposited on the 

 under surface of the crust ? We must suppose that crystallization did not at once extend 

 throughout the whole of any mass of plutonic fluid , but commenced in a layer nearest the 

 refrigerating surface of the superincumbent rocks , and thence slowly extended by accrelions 

 from below. Dr. Lardwer says that » sudden expansion in freezing is particularly con- 

 spicuous in the crystallization of solids which shoot into prismatic forms. The process of 

 crystallization in laboratories is for this reason frcquently attendcd with the fracture of the 

 vessels in which it is conducted. It may be taken as a general truth , to which however 

 there may probably be some exceptions , that bodies which crystallize in freezing undergo 

 the sudden expansion here mentioned , and that bodies which do not crystallize in freezing 

 for the most part suffer a sudden contraction" (1). A priori , therefore , it might be pre- 

 dicated as probable that the granitic fluid, like water, expands on congelation. This pre- 

 dication seems to be verified by observed phenomena. If the granite contracted on crystalli- 

 zation , the crystals formed at and near the surface of refrigeration would sink, the hot 

 fluid from below would constantly ascend to the surface, and when the central heat of the 

 earth was so much lowered as to allow the crystals to reach the centre of gravity in a solid state, a 

 nucleus would there be formed which would gradually increase until the globe was solidi- 

 fied from the centre to the circumference ; or , at all events, no permanent solidification 

 would take place until the temperature of the whole fluid was reduced to the point of 

 congelation (2). Under such conditions, if the globe were originally a homogeöeous fluid, 



matter in different shapes , and which so often assirailates her most diverse processes in the phenomena which re- 

 sult from them. We may seek to isolate particular processes» set the stamp of a name and a theory upon them 

 and extend their exclusive dominion , but still the ministers of nature work togelher and ia harmony , or rather 

 in them the unity of the absolute will still manifest itself. We term some rocks plutonic and some volcanic , and 

 presently we are forced to say that they pass by insensible gradations into each other. As our kuowlerfge extenda 

 all this will probably appear but a play of words. Restraining speculation , we may consider it as established 

 that the plutonic and volcanic rocks are the produce of the same process ; that congelation and deposition may 

 alike cause the stratified structure ; and tbat by a chemical action , often slight , igneous rocks may , in titu , 

 become assimilated to sedimentary, and sedimentary rocks may, in silu , become assimilated to igneous. Obser- 

 vation alone can reconstruct the history of any given rockt and determina the limits within which transmutations 

 are effected in nature. 



(1) Treatiae on Heat, p. 131. 



(2) This appears in granite to be about the same as that of iron (Dï ia Brche's Report p. 191) which is stated 

 in the table appended to Dr. Labdker's Treaiise on ffeat (p. 415) to be 21G37 F. but which appears by the 

 improved pyrometer of Professor Dakjeli, to be only 2788 6 F. {Penny Cycplocedia Art. Freezing and tnelling 

 Poini8.) It should be observed however that graniles vary very greatJy in fusibiiity. Trappean rocks were found 

 by Sir H. os ia Bbchb to fuse at the same temperature with copper or 1996° F. 



